Interpretive Summary: Global warming due to increased concentration of greenhouse gases, such as CO2, in the atmosphere is a major concern. It has been estimated that agricultural practices contribute about 25% of total anthropogenic CO2 emissions. Soil can act both as source and sink of atmospheric CO2. The CO2 fixed in plant biomass through photosynthesis can be stored in soil as organic C by converting plant residue into soil organic matter after the residue is returned to the soil. While management practices, such as tillage, can increase CO2 emission from soil by disrupting soil aggregates, incorporating plant residue, and oxidizing soil organic C, no-tillage practices and increased cropping intensity can increase soil C storage. Respiration by plant roots and soil microflora and fauna also contribute about half of CO2 emitted from the soil. The CO2 emission from soil to the atmosphere is the primary mechanism of soil C loss and provides an early indication of soil C level when changes in organic C due to management practices are not detectable within a short period.
The effects of irrigation systems and soil and crop management practices were examined on soil CO2 flux, temperature, and water and C contents at the 0 to 20 cm depth from May to November 2005 in the northern Great Plains. Irrigation increased CO2 flux by 13% compared with non-irrigation by increasing soil water content during dry periods in North Dakota. Tillage increased CO2 flux by 37% compared with no-tillage by increasing soil temperature at the site previously under CRP but not at the site that had a history of cultivation. The CO2 flux was 1.5 to 2.5-fold greater with tilled than with non-tilled treatments following heavy rain or irrigation in transitional Conservation Reserve program (CRP) land and 1.5 to 2-fold greater with crops than with fallow following substantial rain in dryland. Nitrogen fertilization increased CO2 flux by 13 to 21% compared with no N fertilization at both sites. The CO2 flux in undisturbed CRP was similar to that in no-tilled crops. The CO2 flux was linearly related with soil temperature and daily average air temperature at the time of CO2 measurement. Soil organic and inorganic C contents were not influenced by treatments.
Although soil C storage was not altered, soil and crop management practices influenced CO2 flux within a short period due to changes in soil temperature, water content, and nutrient levels. Regardless of irrigation, CO2 flux can be reduced from cropland using no-tilled legume crops with no N fertilization compared with other management practices.

Technical Abstract:
Management practices can influence soil CO2 emission and C sequestration in cropland and therefore on global warming. We examined the effects of irrigation systems (irrigated vs. non-irrigated) and soil and crop management practices on soil CO2 flux, temperature, and water and C contents at the 0 to 20 cm depth from May to November 2005 in the northern Great Plains. Management practices were no-till malt barley (Hordeum vulgaris L.) with 67 or 134 kg N ha-1 (NTBFN), no-till malt-barley with 0 kg N ha-1 (NTBON), conventional-till malt barley with 67 or 134 kg N ha-1 (CTBFN), conventional-till malt barley with 0 kg N ha-1 (CTBON), no-till pea (Pisum sativum L.) with 0 kg N ha-1 (NTPON), and no-till conservation reserve program (NTCRP) planting that were applied in a Lihen sandy loam (sandy, mixed, frigid, Entic Haplustolls) in western North Dakota. Similarly, in eastern Montana, management practices were no-till rye (Secaele cereale L.) with 0 kg N ha-1 (NTRON), no-till malt barley with 78 kg N ha-1 (NTBFN), no-till Austrian winter pea with 0 kg N ha-1 (NTPON), no-till fallow with 0 kg N ha-1 (NTFON), and conventional till fallow with 0 kg N ha-1 (CTFON) that were applied in a Williams loam (fine-loamy, mixed Typic Argiborolls). The land in North Dakota was previously under Conservation Reserve Program (CRP) for 20 yr while the land in eastern Montana was under dryland cultivation. Irrigation increased CO2 flux by 13% compared with non-irrigation by increasing soil water content during dry periods in North Dakota. Tillage increased CO2 flux by 37% compared with no-tillage by increasing soil temperature at the site previously under CRP but not at the site that had a history of cultivation. The CO2 flux was 1.5 to 2.5-fold greater with tilled than with non-tilled treatments following heavy rain or irrigation in transitional CRP land and 1.5 to 2-fold greater with crops than with fallow following substantial rain in dryland. Nitrogen fertilization increased CO2 flux by 13 to 21% compared with no N fertilization at both sites. The CO2 flux in undisturbed CRP was similar to that in no-tilled crops. The CO2 flux was linearly related with soil temperature and daily average air temperature at the time of CO2 measurement. Soil organic and inorganic C contents were not influenced by treatments. Although soil C storage was not altered, soil and crop management practices influenced CO2 flux within a short period due to changes in soil temperature, water content, and nutrient levels. Regardless of irrigation, CO2 flux can be reduced from cropland using no-tilled legume crops with no N fertilization compared with other management practices.